1. Field of the Invention
This invention relates to determining the Seebeck coefficient for thermoelectric materials. Particularly, this invention relates to measuring the Seebeck coefficient for high temperature thermoelectric materials.
2. Description of the Related Art
An applied temperature difference across a material causes charged carriers in the material (electrons or holes) to diffuse from the hot side to the cold side. Mobile charged carriers migrating from the hot to the cold side leave behind their oppositely charged and immobile nuclei, resulting in a thermoelectric voltage across the material.
The term, “thermoelectric,” refers to the fact that the voltage is created by a temperature difference. Since a separation of charges also yields an electric field, the buildup of charged carriers on the cold side eventually ceases at some maximum value for a given temperature difference as there exists an equal amount of charged carriers drifting back to the hot side as a result of the electric field equilibrium. An increase in the temperature difference can result in more charge carriers on the cold side and thus yield an increase in the thermoelectric voltage.
The Seebeck coefficient (or thermopower) is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across a given material. The Seebeck coefficient has units of volts per degrees kelvin.
                    S        =                              Δ            ⁢                                                  ⁢            V                                Δ            ⁢                                                  ⁢            T                                              (        1        )            The Seebeck coefficient, S, depends on a material's temperature, and crystal structure. Typically, metals have small thermopowers because most have half-filled bands, including both electrons and holes. Electrons (negative charges) and holes (positive charges) both contribute to the induced thermoelectric voltage thus tending cancel their contributions to that voltage, resulting in a low net voltage. In contrast, semiconductors can be doped with an excess amount of electrons or holes and therefore can have large positive or negative values of the thermopower depending on the charge of the excess carriers. The sign of the thermopower indicates which charged carriers dominate the electric transport in both metals and semiconductors.
Accurate measurement of the Seebeck coefficient is critical for the performance assessment of thermoelectric materials. High temperature (e.g. greater than 500 C) Seebeck coefficient measurement systems are necessary for efficient and progressive development of thermoelectric materials for waste heat harvesting applications. The history and challenges of Seebeck coefficient measurement has been recently reviewed. See e.g. J. Martin, T. Tritt, and C. Uher, J. Appl. Phys. 108, 121101 (2010), which is incorporated by reference herein.
In view of the foregoing, there is a need in the art for improved apparatuses and methods for accurately measuring the Seebeck coefficient of thermoelectric materials. There is particularly a need for such apparatuses and methods to operate at high temperatures (e.g. above 500 C). Further, there is a need for such apparatuses and methods to be simple, non-destructive, efficient, fast and affordable. There is also a need for such systems and methods to be suitable for bulk material samples in a broad range of physical types and shapes. These and other needs are met by embodiments of the present invention as detailed hereafter.